In the early 1990s, Colombian drug cartels had a problem: They had more money than they knew what to do with.

"They were having a very difficult time with just the logistics of laundering millions and millions and millions of dollars every week," says Skip Latson, who worked for the DEA at the time.

So Latson and Bill Bruton, who was a special agent with the IRS, hatched a plan: They'd create a fake, offshore bank catering to the needs of the drug cartel.

Latson and Bruton got their bosses to sign on. Soon, RHM Trust Bank was up and running on Anguilla, an island in the Caribbean.

With the help of a South American informant, the bank started getting customers. The big Colombian cartels would send a fax saying they had cash in New York City or wherever. Someone would arrange a pickup, and the cash would be deposited in an account at the fake bank.

The money laundering world was surprisingly vast and sophisticated, with money hidden in the normal flow of international trade, in payments for things like tractor parts.

Latson and Bruton started collecting phone numbers, names and evidence, and pieced together how it all worked. One particularly effective trick: When some shadowy figure asked to withdraw money, they would stall for a day or so. That would often prompt the true owner of the account to call up and demand his money.

"We picked up more information by being a bad bank who didn't handle money quickly and effectively," Latson says.

The fake offshore bank was so successful, Bill and Skip eventually had to shut it down, for fear their cover might be blown.

In the end, 117 people in four countries were arrested in what became known as Operation Dinero. Usually, the secrecy of the offshore world makes life hard for people on the law-enforcement side. But this time, for a moment, the secrecy worked the other way. It made a sting operation possible.

And, while the operation clearly didn't end the drug trade or money laundering, Bruton and Latson say it at least made drug dealers wary of using banks to launder their money.

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RENEE MONTAGNE, HOST:

Earlier this year, our Planet Money team set up an offshore company to try to get a better idea of what goes on in that secretive world. Turns out, theirs was not an entirely original idea. David Kestenbaum brings us this story of two U.S. special agents who went offshore and set up a fake bank.

DAVID KESTENBAUM, BYLINE: If you're a drug trafficker in, say, Colombia, you've got a peculiar problem. The cocaine you send to the United States, people don't pay with credit cards. They pay with cash, sometimes small bills - tens, fives, ones. And all that currency, it's heavy. Bill Bruton was a special agent with the IRS. He remembers one raid on a drug trafficker's home. There was all this cash.

BILL BRUTON: I think it was in, like, seven or eight suitcases, and it took ten of us to carry it, it was so heavy.

KESTENBAUM: In the 1990s, this was a real problem for the Colombia drug cartels. Skip Latson worked for the Drug Enforcement Administration.

SKIP LATSON: They were having a very difficult time with just the logistics of laundering millions and millions and millions of dollars every week.

KESTENBAUM: Every week?

LATSON: Well, yes.

KESTENBAUM: Which is when Skip and Bill came up with what was arguably a crazy idea.

BRUTON: We're saying, well, let's open up a bank.

KESTENBAUM: A bank that would cater to the needs of the drug cartels. Bill and Skip set up an offshore bank in the Caribbean. It needed a name, and they thought, well, it should definitely have the trust in it - also initials. Bank names have initials. And so RHM Trust Bank was born. With the help of an informant, the bank started getting customers. The big Colombian cartels would send a fax, saying we have cash in New York City or wherever, and we need it deposited in this account. And they'd provide a beeper number.

BRUTON: Whoever's going to pick the money up calls the beeper. The guy calls him back and says meet me at XYZ street corner at 10 o'clock tomorrow morning. And what kind of car are you going to be in? Oh, I'm going to be in a blue Chevy.

KESTENBAUM: The Colombian money laundering world was surprisingly vast and sophisticated. The money was being hidden in the normal flow of international trade, in payments for things like tractor parts. But Skip and Bill started collecting phone numbers and names and evidence, and they pieced together how it all worked.

BRUTON: One of the tricks that I always laughed at when Skip wanted to find out who the true owner of the money was, he would delay by 24 hours in sending in the money.

LATSON: And then we'd get telephone calls and find out, oh, that person comes out of the woodwork when it looks like that the money may be in jeopardy.

BRUTON: We picked up more information by being a bad bank who didn't handle money quickly and effectively.

KESTENBAUM: The fake offshore bank was so successful that Bill and Skip eventually had to shut it down for fear their cover might be blown. In the end, 117 people in four countries were arrested in what became known as Operation Dinero. Bill remembers watching a video of the takedown in Italy.

BRUTON: They showed the police precinct, and there were like 20, 30 of these little teeny international police cars, and all of them scream out of the compound at one time and they go in all different directions like ants. And they showed, X hours later, them bringing all these fugitives or arrested people back in.

KESTENBAUM: Usually, the secrecy of the offshore world, it makes life hard for the law-enforcement side. But this time, for a moment, the secrecy worked the other way. It hid the sting operation possible. Here's Skip.

LATSON: I know one thing: They can't trust a bank now. It could be another government bank.

KESTENBAUM: Do you guys feel like you're winning the money laundering battle, or is it just a matter of setting them back for a few years and they find some other way to do it?

BRUTON: I think we were winning 20 years ago and I think we're winning today, because a win doesn't necessarily mean you stop it. You change the methodology for doing it.

KESTENBAUM: These days, Bill says, a lot of the drug traffickers are having to take their heavy cash and physically carry it, sneak it across the border. Doing it that way, you don't need an offshore bank. You just need a truck. David Kestenbaum, NPR News. Transcript provided by NPR, Copyright NPR.